1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst.
2. Description of the Related Art
It has been known an exhaust gas purifying apparatus for an internal combustion engine with multi-cylinders, in which an engine air-fuel ratio is controlled to a stoichiometric point, wherein a three-way catalyst is arranged in an exhaust passage connected to each cylinder, to bring nitrogen oxides NO.sub.x, hydrocarbon HC, and carbon monoxide CO, which are included in the exhaust gas, into contact with the three-way catalyst, to thereby reduce all components NO.sub.x, HC, and CO simultaneously. Note that an air-fuel ratio of an air-fuel mixture in a combustion chamber of an internal combustion engine is referred to as an engine air-fuel ratio, hereinafter.
On the other hand, a low fuel consumption rate is desirable, and thus it is desirable to make the engine air-fuel ratio as lean as possible. However, if the engine air-fuel ratio is made to be lean with respect to the stoichiometric point, the above-mentioned exhaust gas purifying apparatus cannot reduce NO.sub.x sufficiently.
To solve this problem, Japanese unexamined patent publication No. 4-365920 discloses an exhaust gas purifying apparatus for an internal combustion engine with multi-cylinders, the engine having first and second cylinder groups. The purifying apparatus is provided with: an engine operation control device to continuously make each cylinder of the first cylinder group perform a rich engine operation in which the engine air-fuel ratio is set rich with respect to the stoichiometric point, and to continuously make each cylinder of the second cylinder group perform a lean engine operation, in which the engine air-fuel ratio is set lean with respect to the stoichiometric point; a first exhaust passage connected to each cylinder of the first cylinder group; a second exhaust passage connected to each cylinder of the second cylinder group and different from the first exhaust passage; an ammonia synthesizing catalyst arranged in the first exhaust passage for synthesizing ammonia NH.sub.3 from NO.sub.x in the inflowing exhaust gas; an interconnecting passage interconnecting the first exhaust passage downstream of the NH.sub.3 synthesizing catalyst and the second exhaust passage; and an exhaust gas purifying catalyst arranged in the interconnecting passage to react NO.sub.x and NH.sub.3 flowing therein with each other to thereby reduce NO.sub.x and NH.sub.3 simultaneously. In this exhaust gas purifying apparatus, the fuel consumption rate is reduced by increasing the number of the cylinders of the second cylinder group in which the lean engine operation is performed, while NO.sub.x is sufficiently reduced by introducing the exhaust gas from the first cylinder group in which the rich engine operation is performed to thereby synthesize NH.sub.3, and by reacting this NH.sub.3, and NO.sub.x, from the second cylinder group at the exhaust gas purifying catalyst.
The exhaust gas purifying catalyst may be a catalyst having a zeolite carrying cobalt Co, copper Cu, Nickel Ni, or iron Fe. However, the inventors of the present application have found that such a catalyst sufficiently reduces NO.sub.x and NH.sub.3 simultaneously only when the temperature of the exhaust gas entering therein is within a specific temperature range, which is referred to as a purifying temperature range hereinafter. In the actual engine operation, however, the inflowing exhaust gas temperature varies outside the purifying temperature range. When the inflowing exhaust gas temperature is out of the purifying temperature range, the above catalyst would not reduce NO.sub.x and NH.sub.3 simultaneously sufficiently. An additional device to cool or heat the exhaust gas flowing into catalyst may keep the inflowing gas temperature within the purifying temperature range, but such an additional device may complicate the arrangement of the purifying apparatus, and may be costly.